The Granny Smith apple is named after Maria Ann Smith, an Australian orchardist who discovered a chance seedling growing in a compost heap on her farm. The seedling was likely a cross between a domestic apple and a European crabapple. Smith noticed the fruit was exceptionally crisp and tart. Unlike many other apples of the time, the Granny Smith has a thick skin and high acidity, which act as natural preservatives, giving it an incredibly long shelf life. By the 1960s, it became a global supermarket staple. Chemically, its tartness is due to high levels of malic acid. It is also famous for not browning as quickly as other varieties when sliced, a trait caused by lower levels of polyphenol oxidase enzymes.